lbcd 3.5.1

This is a small bug-fix release of this load-balancing system status
daemon. The primary purpose of the release is to merge the various fixes
to the new systemd unit files that I made in the Debian package after a
broader code review. This release also fixes a long-lived oversight and
sets lbcd's listening sockets to close-on-exec so that they're not
inherited by external weight commands (already handled under systemd by
socket activation) and improves the lbcd man page.